The present invention relates to a catalyst, and associated method, for purifying exhaust gases. More particularly, it relates to the catalyst, and associated method, for efficiently purifying nitrogen oxides in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than that required for oxidizing carbon monoxide and hydrocarbons therein.
Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbon and nitrogen oxides (NOx) produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., a low oxygen concentration and an A/F ratio below about 14.6, catalyst containing palladium or platinum are able to efficiently convert all three gases simultaneously. That is, the carbon monoxide and hydrocarbons are oxidized to carbon dioxide and water and the NOx is reduced to nitrogen. Hence, such catalysts are referred to as “three-way” catalysts. A well known example of the three way catalyst is composed of a heat-resistant substrate of cordierite or the like and a porous support layer of gamma-alumina, which supports noble metals such as platinum, rhodium or the like.
The fuel economy of internal combustion engines is becoming an issue of concern because of the high volume of CO2 emitted and also because of the rapid consumption of valuable oil reserves. These concerns are leading to stiffer requirements for fuel economy and reduced emissions. The gasoline direct injection (GDI) engine is one approach to addressing these problems, whereby the engine is operated very lean for most of the duty cycle. Under this “lean-burn” condition the A/F ratio is greater than about 14.6, i.e., resulting in a high oxygen concentration. While the aforementioned conventional three way catalysts can oxidize or reduce carbon monoxide, hydrocarbon and nitrogen oxides to purify exhaust gases when the air-fuel mixture is at the stoichiometric air-fuel ratio, they however, do not exhibit sufficient purification performance for nitrogen oxides in these “lean burn” atmospheres containing excess oxygen. Under these circumstances, the development of both a catalyst and a purifying system capable of efficiently purifying nitrogen oxides even in the atmosphere containing excess oxygen has been demanded.
U.S. Pat. No. 5,948,376 (Miyoshi) discloses one such catalyst/purifying system for purifying exhaust gases in oxygen rich atmospheres. Disclosed therein is a catalyst comprising a carrier material, such as gamma alumina, with a NOx storage component loaded on the carrier material, the NOx storage component including at least one member selected from the group consisting of alkaline-earth metals, alkali metals and rare-earth elements. The carrier material additionally has a noble metal catalyst ingredient loaded on the porous support. This catalyst functions in the following manner: (1) under a lean burn atmosphere in which oxidizing concentrations are above a stoichiometric point that is required for oxidizing components to be oxidized in the exhaust gas, a majority of the nitrogen oxides in the exhaust gas are adsorbed to the NOx storage component on the carrier material; (2) when the exhaust gas is temporarily changed from lean burn to fuel-rich in which oxygen concentrations are below stoichiometric point, the adsorbed nitrogen oxides are released and chemically reduced by a reaction with the hydrocarbons and carbon monoxide in the exhaust gas.
Specifically, Miyoshi discloses preferred embodiments that utilize a cordierite honeycomb substrate upon which is placed an Al carrier material containing CeO2, a NOx storage component of Ba, La, Li, K or Na, and a noble metal catalyst of Pt or Pd.
The NOx adsorption, in the form of nitrate formation, which typically takes place under the lean conditions occurs in a low temperature window of from ˜200° to 550° C., depending on the alkali and/or alkaline earth metals in the coating. NOx desorption and reduction to N2 occurs in the same temperature window, but under the aforementioned rich conditions. Greater than 90% of the life of the system is spent in this temperature regime. Periodic sulfur regeneration is also needed as sulfur in the fuel leads to sulfate formation more readily than nitrate formation. Sulfates are also more stable than nitrates. Sulfur adsorption reduces the number of sites available for nitrate formation. Sulfur regeneration typically occurs under these rich conditions in a higher temperature window of from about 600° C. to 800° C., likewise depending on the alkali and/or alkaline earth metals in the coating. Less than 10% of the life of the system is spent in this high temperature regime. While the performance of NOx trap systems is thought to be quite stable at lower temperatures, temperatures in excess of 800° C. are thought to result in the reduction in the performance of the NOx adsorber.
The inventors of the present invention have discovered that one disadvantage of the present catalyst/purifying systems is that, most of the alkali metals readily react with cordierite within the temperature range of interest for NOx adsorber use. For example, potassium, which is especially desirable as an alkali adsorber material, because of its wide window of operating temperature, appears to very readily react with cordierite which draws the potassium out of the high surface area washcoat, thereby preventing it from performing its adsorber function. Further, the potassium reacts with the cordierite to form relatively high CTE phases that make the substrate and thus the catalyst system much less thermal shock resistant. The present invention has been developed in view of the aforementioned circumstances. Accordingly it is an object of this invention to provide a catalyst for purification of exhaust gases that is not subject to degradation at those higher temperatures experienced when the engine is operating under lean conditions; i.e., where the oxygen concentrations of the exhaust gases are at the stoichiometric point or more.